/Engine/Plugins/FX/Niagara/Source/Niagara/Public/NiagaraMeshRendererProperties.h
This enum decides how a mesh particle will orient its "facing" axis relative to camera. Must keep these in sync with NiagaraMeshVertexFactory.ush
enum ENiagaraMeshFacingMode { Default = 0, Velocity, CameraPosition, CameraPlane, }